Ultrasonic imaging machines are popular for a variety of diagnostic and therapeutic medical and clinical procedures, e.g., cardiovascular diseases, gynecologic and obstetric applications, tumor studies and pulmonary diseases. One of the most widespread uses of ultrasonic imaging equipment has been in connection with early pregnancy diagnosis and the evolution of fetal development and well being. Medical personnel rely on the generated images to observe and study the growth, development and movement of the early conceptus and subsequently the fetus. Such ultrasonic monitoring also will enable the detection of abnormalities. The College of Obstetrics and Gynecology now recommends that women undergo routine prenatal ultrasonic evaluations, and routine ultrasonic imaging has, therefore, become the norm for proper prenatal care.
Ultrasonic imaging machines, however, have their limitations and may not be completely risk-free. Current ultrasonic devices require the direct transmission of high frequency, high power, sonic signals, typically in the range of 3 to 7 megahertz. In the obstetrics setting ultrasound is directed into the mother's womb transabdominally or, via a probe inserted into the vagina, transvaginally to generate an image with sufficient resolution and clarity to allow proper evaluation. Laboratory studies on tissue exposed to ultrasound, however, suggest that prolonged exposure to such high energy waves may damage fetal or maternal tissue.
A typical prior art ultrasonic echographic imaging assembly is shown in FIG. 1. A probe 100 carries a plurality of transducers 101 in a known geometric array. Wave/pulse generator 102 drives transducers 101 to simultaneously transmit an ultrasonic signal toward the target to be imaged (not shown). Echo signals from the target are received by transducers 101 and communicated to a signal processing apparatus, such as computer 103, through analog-to-digital converter 104. The transmitted signal also is communicated to the signal processing apparatus, either directly from wave/pulse generator 102 through converter 104 or, more typically, from transducers 101.
Processing of the transmitted ultrasonic signals and received echo signals typically involves autocorrelation of the transmitted and received signal for each transducer, as indicated by autocorrelator box 106, and may include cross-correlation of the data for each auto correlation with the data for each other autocorrelation, as indicated by cross-correlator box 107. The autocorrelated and cross-correlated data is then both stored in a data matrix 108. A three-dimensional image is available through the use of such an array. The time of arrival of the echo signals will contain two components, namely, range and azimuthal position.
The data yielded from the auto- and cross-correlation is referred to, for example, in radio astronomy, as the complex visibility function comprised of "visibility amplitude" data, A.sub.ij.sup..tau., and "visibility phase" data, .phi..sub.ij.sup..tau.. See, e.g., Thompson, et al., "Interferometry and Synthesis in Radio Astronomy", 1986 John Wiley & Sons, p. 14. While these data are in time domain, the convention is to describe them with equivalent variables in the frequency domain. The frequency domain is described herein by the usual convention, the Fourier transform of the time domain is the Fourier transform of the frequency domain. As is well known in the ultrasonic imaging field, the visibility amplitude data is much more reliable than the visibility phase data, which is severely corrupted by noise, such as complex side lobes, systematic phase noise, and calibration problems. Calibration problems, particularly those due to phase differences from the propagation of ultrasonic wave within an inhomogeneous medium lead to so called "phase aberration," often considered to be the greatest obstacle to good echographic imaging.
Two broad approaches have been taken to the phase aberration problem. One is to disregard or not use the phase data because it can contribute so much noise that its use does not enhance the signal-to-noise ratio or the resultant resolution and dynamic range of the images produced. The other broad approach is to provide a system which attempts to correct for phase aberration, for example, by the use of filters, time delays or other approximations.
When phase data is disregarded, imaging essentially is effected in prior art ultrasonic apparatus by mapping the visibility amplitude data, as indicated by mapper box 111. Mapper 111 combines the data with position information as to transducer location to synthesize an aperture corresponding to the probe transducer array. Once the image is mapped, the image of the target can be displayed on output device 112, which is advantageously a video display terminal.
Unfortunately, the visibility amplitude data still contains considerable noise. Equipment calibration and side lobe effects contribute significant noise, and the image which results has a resolution and dynamic range well below that which would be optimal.
When phase data is used in ultrasonic imaging, one of the most common approaches to the reduction phase aberrations is the use of adaptive reduction of phase aberration based upon cross-correlation techniques. U.S. Pat. No. 4,817,614 to Hassler et al. and U.S. Pat. No. 4,835,689 to O'Donnell both employ adaptive reduction. Cross-correlation of signals from multiple transducer arrays are employed to enable time delay corrections to be inserted at each transducer which seek to reduce phase aberration. This approach is reminiscent of "rubber mirror" approaches in optics. Such modeling schemes, however, are only ad-hoc attempts to derive a true, noiseless visibility phase. Their results can vary from scan-to-scan, time-to-time, organ-to-organ and patient-to-patient, and thus are of limited replicability and utility in a clinical context. See also, e.g., companion technical articles, O'Donnell et al. "Phase Aberration Measurements in Medical Ultrasound: Human Studies", Ultrasonic Imaging, Vol. 10, pp. 1-11 (1988); O'Donnell et al., "Aberration Correction without the Need for a Beacon Signal", IEEE Ultrasonics Symposium, pp. 833-837 (1988); O'Donnell et al., "Phase-Aberration Correction Using Signals from Point Reflectors and Diffuse Scatterers: Measurement", IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 35, No. 6, pp. 768-774 (1988). See, also, Hayakawa "Multifrequency Echoscopy for Quantitative Acoustical Characterization of Living Tissues", J. Acoustical Society of America, Vol. 69 (6), pp. 1838-1840 (1981) where an approximation of the attenuation coefficient in human tissue is developed.
In an article by Somer et al., "Real-Time Improvement of both Lateral and Range Resolution by Optical Signal Processing", Ultrasonics Symposium Proceedings pp. 1002-1005 (1977) an ultrasonic image enhancing process is described in which lateral and axial resolution is improved by using coherent optical filtering in a cross-correlation process. This is an optical approach to achieve an approximate phase aberration correction.
In U.S. Pat. Nos. 4,604,697 and 4,553,437, both to Luthra et al., a hybrid image is produced from the vector addition of amplitude and phase data from an array of transducers at a plurality of frequencies. The overall image is produce by adding partial images. In U.S. Pat. No. 4,586,135 to Matsumoto side lobe reduction is employed utilizing phase data to provide a holographic data set for reconstruction by a synthetic aperture technique.
Auto and cross-correlation also have been used in U.S. Pat. No. 4,397,006 to Galbraith to determine digital time domain filter parameters for noise reduction in seismographic teachings.
Another common approach to enhancement of the signal-to-noise ratio has been to employ relatively high power and high frequency ultrasonic waves in medical applications. Increase in power tends to increase the signal strength relative to the noise. With respect to reducing noise by increasing frequency, the expansion of an ultrasonic beam is inversely proportionally to the beam frequency. Thus, in the biomedical field high- frequency ultrasonic imaging devices are often required to attempt to minimize beam expansion and maximize image resolution. However, high-frequency beams are attenuated substantially as they pass through body tissues, which also reduces signal-to- noise ratio. In fact, the resolution of ultrasound images at depths in bodily tissues greater than about 10 centimeters is so poor as to have limited clinical value. At greater depths, low frequency beams experience less attenuation. But, as the frequency drops the beam expands and the resolution decreases. Thus, frequency compromises are often used to try to optimize resolution for the particular application, but in general, higher than optimal frequencies are used at the sacrifice of depth and clarity of imaging.
Present ultrasonic echography or imaging devices are limited, therefore, in the resolution quality of the images obtained. Images are generally very fuzzy and shadow-filled, and tissue details that require accurate imagery for detection often go undiscovered. Use of current ultrasonic equipment, therefore, requires considerable experience and skill, and even with such experience and skill, the information which can be gleaned from echographic images is very limited and requires subjective interpretation.
The result is that even the high frequency, high power, ultrasonic apparatus most commonly used in medicine today is capable of generating only fuzzy images of tissue targets located at a radial distance in the patient's body of only about a few centimeters from the transducer bank.
Still other attempts have been made to enhance the clarity or resolution of ultrasonic images, but only limited success has been achieved. In U.S. Pat. No. 4,478,085 to Sasaki, the thickness of the ultrasonic transducers was varied over the array to try to minimize beam expansion. U.S. Pat. No. 4,470,305 to O'Donnell employs an annular array of ultrasonic transducers and time delayed pulses to simulate a horn transducer having a sharp focus in the near field. Using this system improved focusing can be achieved up to 20 centimeters, but imaging is accomplished at 3 MHz. The improvement in focus at depth is accomplished in the O'Donnell patent by using variable received signal gains to try to reduce the side lobe noise in the images.
In U.S. Pat. No. 4,677,981 to Coursant, improvement in the ultrasonic echographic image focusing is attempted by using polarization characteristics of the ultrasonic transducers. The disadvantage of this approach is the absence of the initial polarization information and a lack of total intensity. This approach adds little to significantly improve dynamic range and resolution of the ultrasonic images.
Variable frequency ultrasonic scanning also has been used, e.g., U.S. Pat. No. 4,442,715 to Brisken et al., and pitch variation is employed in the device of U.S. Pat. No. 4,664,122 to Yano. Doppler shift also has been employed to detect motion of scanned targets, for example, as is taught in U.S. Pat. No. 4,509,525 to Seo.
Finally, in a published abstract of a paper that was net given or published, I suggested that in underwater acoustic imaging linear and nonlinear imaging techniques could be used to aid in recovery of phase observables for increased dynamic range and image accuracy. I also suggested that techniques from imaging disciplines such as optics and radio astronomy might be applied. Cohen, "Phase Recovery and Calibration with Underwater Acoustic Arrays", J. Acoustical Society of America, Sup. 1, Vol. 82, pp. 574-575 (1987). The techniques which might be applicable, how they might be applied and their suitability for medical imaging is not set forth in the abstract.
While modest improvement has been achieved with prior image enhancement techniques, an ultrasonic echography device producing images having good quality has not been achieved. The high levels of noise associated with such signals has rendered consistent, accurate imaging impossible. Accordingly, there is a need for a safe echography apparatus capable of generating high resolution imaging with a high dynamic range.